Unit 5 - Geophysics Flashcards
Name some information that a geologist can gain from geohpysical data?
- geometry of rock bodies that comprise petroleum system
- distribution of reservoir properties (its internal structure)
- size and distribution of aquifer
- The presence of internal compartments or disruptions of the reservoir (such as faulting)
- Possibly, direct indications of hydrocarbons
what is the main motivation to undertake geophysical surveys?
- develop and image of the subsurface geology
What is the secondary motivation of geophysical surveys?
- determine properties of the rocks and to detect their contained fluids
What are some of the important physical properties of rocks to develop an image of the subsurface?
- The sonic velocity (speed of passage of acoustic waves)
- The density
- The resistivity (or its converse, the conductivity)
- The magnetic compliance, and/or the natural magnetism
What are the four primary geophysical investigation methods?
- gravity
- electrical
- magnetic
- sonic
Describe gravity methods ?
- based on the fact, that changes in density in parts of the earth can be detected because of their effect on the gravity field
- gravitational effects associated with mass decline with distance
- in practice this method is used to primarly to detect big rock bodies that have a significant anomaly in density ( e.g. salt diapir)
Describe Electrical methods?
- based on fact that rocks have varying resistivities
in practice: an electrical potential (voltage) is applied across widely-separated electrodes located at the ground surface
→ another electrode pair is used to determine the electrical potential at intervening sites
- Source rocks can have lower resistivities because of their high carbon content
- electrical techniques are not commonly used for reservoir studies.
what are seismic waves?
- form of an acoustic wave
→ have periodic, oscillatory motions
- can be created naturally ( e.g. earthquakes) or be intentionally induced as part of an investigation
What affects the passage of seismic waves?
- rock pattern
- distribution of fluids
What are the two main categories of seismic techniques?
- seismic refraction
- reflection seismology
What is seismic refraction?
- goal is to determine position, velocity and density, of rock bodies
- effective at “seeing” situations where lateral dimension is very much greater than the depth
- depth resolution/accuracy is rather poor (at least for problems of interest to Petroleum Engineering)
- this approach has only limited applications to reservoir problems
What is reflection seismology or reflection seismic?
- sonic (vibrational) energy is propagated into the ground with the approach to detect the sonic energy that is reflected back to surface
→ this data is used to create an interpretation of the subsurface
- normal approach is to display the data in a way that looks similar to a geological cross section
What are the two types of body waves?
P-waves – ComPressional,longitudinal,Pressure,orPrimarywaves(Push-Pull)
S-waves – Shear, transverse, tangential, or Secondary waves (Side-to-Side)
What is a P-wave?
- elastic body wave in which the particle motion involves oscillations in the direction of propagation of the wave
What is an S-wave?
- an elastic body wave in which the particle motion involves oscillation in planes that are perpendicular to the direction of propagation of the wave
What is divergence?
- degrading of energy of a wave while it progrades due to the fact, that it has to move more and more particles with expansion due to a bigger radius
Why do higher frequency waves loose their energy more readily over a given distance than low energy waves?
because there are more oscillations per second for the higher-frequency wave, and each oscillation has a “loss” attached to it
Which wave travels faster? A P-wave or an S-wave?
P-wave
What do you call mode conversion?
-if wave types convert (e.g. p-wave to s-wave and vice versa)
What is a seismic trace?
- the amplitude versus time plot of the signal that is received at the surface
Describe a seismic investigation!
- Mechanical energy is injected into the ground
- The acoustic waves that are generated travel through the rocks
- Some of the energy is reflected from a variety of interfaces and returns to the surface at varying times
- There, the incoming waves are detected by sensors(geophones or hydrophones) and the data recorded
- computer algorithms manipulate the data and produce images that show spatial distribution of physical properties of the rocks
The first four steps are called acquisition, and the fifth step is called processing (and this merges with a task called interpretation).
How do you call a seismic receiver onshore?
- geophone
How do seismic receivers work?
- they detect pressure changes caused by compressional waves and convert them to electrical signals
What is filtering?
- recorded data is subjected to a frequency filter to make interpretation easier
Main objectives of seismic interpretations are to determine?
- Two-way travel times and/or depths to rock interfaces
- The dip of interfaces
- The location of discontinuities in rock interfaces (faults)
- Changes of stratigraphy (due to lateral facies changes) or unconformities
- Rock and/or fluid properties from seismic velocities or impedances within layers
What is a check-shot survey or a velocity survey?
- sonic source is placed at the surface, and a geophone is progressively lowered along the wellbore to determine the actual average compressional velocity from the surface to any depth
What is tomography?
-a medical technique to reconstruct a section through a body from measurements around the body
Lithology can sometimes be predicted from seismic data by ?
Using the amplitude
Wireline logs are measured from which level?
- rotary table
